Traditionally a vehicle safety restraint comprises seat belt webbing which passes around the torso and around the lap of a vehicle occupant to secure him in a vehicle seat. One end of the seat belt webbing is attached to a releasable buckle and the other end is wound onto the rotatable spool of a retractor. A modern retractor allows a vehicle occupant to move, for example to lean forward to reach radio controls or a glove compartment, provided the movement is relatively gentle. However in a crash when the occupant is subject to a sudden deceleration, a crash sensor causes the retractor to lock to prevent any further payout and to securely restrain the occupant.
In high velocity crashes the force exerted by the seat belt webbing on the vehicle occupant is extremely high and can itself cause injury. It has been proposed to introduce load limiting arrangements so as to controllably absorb some of this force felt by the occupant.
Load limiting arrangements can be introduced into any part of the webbing force path. For example a load limiter may be incorporated into the buckle mounting, or into the retractor itself, such as in the winding of the spool. The energy can be absorbed by bending pieces of metal, e.g. incorporating crush tubes or torsion bars into the system. Other known arrangements involve hydraulics, e.g. extrusion of fluids.
Some load limiting arrangements are described in co-pending GB applications 97 21 918.2 filed Oct. 14, 1997, 97 21 919.0 filed Oct. 14, 1997, 97 21 921.6 filed Oct. 14, 1997, 97 21 922.4 filed Oct. 14, 1997 and 97 21 924.0. filed Oct. 14, 1997.
A problem with the known systems which rely on bending metal or torsion bars is that the force absorbed is the same at low speeds, with small, light occupants, as at high speeds with large occupants. This constant force means that the large occupant, or the high speed crash, causes more payout of seat belt webbing than for low speeds, and in worst case situations the forward movement of the occupant, despite the belt restraint, can be so excessive as to result in an impact with an interior part of the vehicle causing injury.
Hydraulic systems have been suggested in which a feedback loop is provided so that a higher force can be absorbed with a higher speed of movement. Such systems are described in co-pending GB application 97 21 918.2 filed Oct. 14, 1997, particularly with respect to FIGS. 1 to 3. In these arrangements the absorbed force rises exponentially with the speed of movement of the occupant (generally the force is proportional to the speed to the power n, where n is between 1 and 2, depending upon the fluid properties and the design of the feedback valve). However this rapidly increasing force absorption is too severe and at very high speeds the force felt by the occupant is likely to be excessive.
Using different valve arrangements, for example profiling the valve pin, or changing the internal shape of the valve housing can provide a variety of force-speed relationships, for example a force absorbing curve which rises and then flattens with increasing speed, or a stepped arrangement, but the valve must be individually adapted to the expected crash pulse, and in practice only an average situation can be catered to.